The present invention relates to the field of laser communications. More specifically, the present invention relates to the field of laser directional control through refractive optical elements.
In the optical propagation of communication signals, the conventional approach is through the use of glass fiber xe2x80x9ccables,xe2x80x9d i.e., fiber optics. Fiber optics allows the propagation of clean, noise-free signals of high bandwidth that are effectively immune to electromagnetic interference. In an exemplary downlink application, a trunk (i.e., multifiber) cable is run to a distribution head, where each of the individual downlink fibers of the trunk cable is spliced to a downlink fiber of a service (i.e., single fiber each direction) cable. The service cable is then routed to a local apparatus. A reverse approach occurs in the uplink direction, where applicable.
A communication signal distribution system for a housing or business tract may utilize the exemplary scheme described above. The trunk cable is coupled at a local distribution head into service cables for each building, with the local apparatus being an optoelectronic transceiver at each building configured to convert between the optical downlink and uplink signals and internal electronic (wired) signals. The wide bandwidth possible with optical signals allows multiple simultaneous television, music/news, telephone, fax, hi-speed data (computer), security, and other signals to be received and/or transmitted.
One disadvantage of the above housing or business tract scheme is that, to meet contemporary zoning codes and/or construction practices, the service cables from the distribution head to each building may need to be buried. Cable burial poses problems of cost, implementation, and upkeep/repair. It may cost from thousands to millions of dollars per kilometer to bury cable. Usually, the more urban or built-up the area, the greater is the per-kilometer cost of cable burial. In many cases, the cost of cable burial is prohibitive.
Additionally, a disadvantage of cable burial is the burial itself. It may not always be practical or even possible to bury a cable (optical or wire), as where a cable may have to cross third party property, a canyon or gorge, a river or lake, etc.
Buried cables are subject to damage. Cables may be damaged by street repair or utility crews during maintenance or installation of services. Similarly, individuals may damage cables during the installation, maintenance, or repair of pools, spas, irrigation systems, landscaping, septic and sewage lines, etc. Those skilled in the art are well familiar with the problems that plague buried cables.
Another disadvantage of using fiber optics is the installation of the optical fibers themselves. While an acceptable propagation medium, an optical fiber requires a labor intensive installation. Careful and time-consuming alignment between the optical fiber and a terminating laser or photodiode is required. This contributes significantly to the overall cost of a fiber optic system.
Cable burial and fiber termination problems may be eliminated by using an aerial or spatial transmission scheme. In this approach, a collimated beam is transmitted directly through the atmosphere (or through space) from a transmission location to a reception location. A reciprocal beam may likewise be aerially or spatially transmitted for bidirectional communication.
Aerial transmission has problems with collimation and penetration. Typically, an aerial laser transmission scheme uses some sort of optical collimator (e.g., a telescope) to produce a highly collimated beam from a laser at the collimator""s focal point. In a distribution head serving a large number of clients, the use of individual collimators leads to a costly complexity both in materials and in installation. Since each collimator is essentially a telescope, many collimators means many tubes, many lenses, and many mounts, all of which add to the system cost. Since each collimator must be individually aimed at its target, the use of many collimators involves a complex and time-consuming installation procedure.
The transmitted aerial laser beams of any system serve no function unless they are received. If the transmissivity of the atmosphere is such as to absorb the beam prior to reception, then the beam is worthless and the link is broken. Typical solutions for transmissivity problems are improvements in collimation and increases in power. Both solutions serve the same function, i.e., to increase the flux density at the receiver.
If a given photoreceptor has a specific lumen threshold, then the photonic flux falling upon that photoreceptor (the received flux) must be above that specific lumen threshold to be significant. Two ways in which the received flux may be increased include an improvement in collimation and an increase in transmitted optical power.
A given laser beam has a specific total photonic value. As the laser beam diverges, this total photonic value is spread over an ever-increasing area, i.e., the flux density decreases. At the target distance the beam therefore exhibits a specific received flux density. With an improvement in collimation, the beam has a smaller diameter at the target distance and the received flux density is increased. Presuming for discussion purposes that the distance between transmitter and receiver is constant, an increased flux density permits the reception of an adequate signal with a reduced atmospheric transmissivity. Similarly, with an increase in transmitted flux, the received flux density is increased. Again, an increased flux density permits the reception of an adequate signal with a reduced atmospheric transmissivity.
Likewise, for a given atmospheric transmissivity, an improvement in collimation or an increase in transmitted flux, the received flux density is increased and the distance between transmitter and receiver may be increased while maintaining an adequate reception signal.
What is needed, therefore is a device allowing individual, simultaneous, and cost-effective control over the collimation and transmitted flux density of a plurality of laser beams in a distribution head.